After they had failed the writing exam that was required of all students before graduation, fifteen students were placed in a class in which they received additional tutoring. After completing this class, all fifteen students retook the exam and earned higher scores. Their success was held up as an example that the class curriculum really works. What source of internal invalidity has not been considered?

Regression.

Explanation:

As per the question, the source of internal invalidity that has not been taken into consideration here would be 'regression' as the conclusion is not based on reliable research and the key reason for research not being reliable is that it selects the subjects(fifteen students who failed) based on extreme characteristics or scores('they all earned higher scores'). Thus, research becomes internally invalid as the deduction failed to consider 'regression' aspect and hence, could not proffer a justifiable and reasonable claim.